In the RF transmission of digital information, sampled data sequences are converted to analog signals and processed, subsequently, by various operations containing unwanted nonlinearities. The primary source of nonlinearity is the power amplifier (PA). Nonlinear behavior of the PA (or other devices) can be compensated using digital predistortion. That is, the correction signal is a sampled sequence applied prior to the PA. The correction signal, denoted by xDPD(nT), is represented as a set of higher-order sub-signals corresponding to nonlinear modes in the transmitter.
The nonlinear behaviour of the PA transfer characteristics can be classified as memoryless or memory-based. For a memoryless nonlinear device, the nonlinear modes are functions of the instantaneous input value, x(t), only. In contrast, for a PA exhibiting memory effects, the nonlinear modes are functions of both instantaneous and past input values. In general, memory effects exist in any PA; however, the effect becomes more apparent when the bandwidth of the input signal is large. As a result, the correction of memory effects is becoming increasingly more important as wide bandwidth modulation formats are put in use.
Accordingly a need presently exists for a system and method for correcting distortion in power amplifiers and especially distortion due to memory effects.